This invention relates to a method of filling a narrow, flat display system cavity with liquids highly volatile in nature and prepared to contain particulate matter evenly disbursed throughout the liquid volume. Fluid suspensions used in the context of this invention include liquid colloidal suspensions and emulsions with particles having the properties of responding to an applied electric field across a thin portion of the fluid suspension to affect intensity of light transmitting through the fluid suspension.
Such devices are known as light valves and are usually constructed of two transparent flat substantially parallel walls which are separated by a relatively small distance, generally on the order of 0.5 mil to 50 mils, and are sealed around the periphery to form an enclosed hermetically sealed cell. Thin conductive transparent coatings are applied on the interior or exterior surfaces of the walls and the cell is filled with a fluid suspension containing particles affected by the application of an electric field. Light valves can be used to allow or disallow transmission of light through the cell.
In the usual case, when a voltage is applied to the conductive coatings of the light valve (i.e., across the fluid suspension) the particles align perpendicular to the walls of the cell in a manner that the suspension appears transparent to the light passing through. When no voltage is applied, the particles are randomly disbursed by Brownian motion and in this condition the fluid suspension appears dark, extinguishing visible light rays attempting to pass through the suspension.
In filling the cavity of the light valve cell certain problems are encountered which cause a lack of uniformity or homogeneity over the surface area of the cell. One such problem is caused by the basic physical properties of volatile liquids which are used to make up the fluid containing the suspended particles. Volatile liquids with high vapor pressure tend to vaporize easily into gaseous form causing bubbles or voids when they are drawn into the cavity by a vacuum applied at an end opposite to a reservoir of the fluid which is fitted to allow the fluid to enter the cavity. Once gas voids or bubbles are formed they are difficult to eliminate within the cavity.
A second problem arises during filling a cavity under pressure by pumping in the fluid suspension. The thin transparent glass plates used as flat walls in the light valve tend to bow outward from the slight difference in pressure between the liquid suspension within the light valve and the atmospheric pressure outside. This problem becomes more severe the larger the surface area of the light valve cell becomes.
This bowing or distortion of the surface plates leads to a light valve cavity that is not uniform in thickness. The plates will be separated more at points away from the fixed boundaries. Therefore, the thickness of the suspension through which the light passes will differ throughout the cell cavity. Accordingly, the optical density will vary from point to point within the light valve cell.
It is further recognized that such variation in the thickness of the fluid suspension causes variations in the electric field gradient which itself causes pronounced variations in light transmission characteristics from point to point within the cell. Bowing and distortion of the glass walls can also cause fracturing of the walls of the cell, rupturing of the seals or other serious damage.
Therefore, it is important that the cell walls be returned to, or kept in approximately the same flat, parallel relationship as before the fluid was added to the cell. This will protect against physical damage and distortions as well as provide that the optical properties of the light transmission through the cell be uniformly controllable.
One method at solving this particular problem is presented in the U.S. Pat. No. 3,744,126 by Forlini et al. This patent discloses a method utilizing a single fill port for both filling the cell with a fluid and withdrawing some of the fluid later to reduce the internal pressure. The method presented also allows for the existence within the cell of small quantities of vapor or air pockets to exist, although it is claimed that these small voids cause no adverse affect to the cell's performance.
U.S. Pat. Nos. 3,742,600 and 3,912,365 by Lowell present a method for inserting small spherical beads into the cavity of a light valve, where such spheres have a diameter equivalent to the desired spacing between the plates of the light valve. The small non-deformable beads therefore, when distributed throughout the suspension, tend to prevent the walls from coming closer together than the preferred distance of their diameter when a pressure difference between the liquid within the cavity and the surrounding environment is such as to tend to compress the content of the cavity. This situation would occur if, in order to reduce the positive differential in pressure which would cause outward bowing of the plates, the liquid mixture was subjected to a slight vacuum thereby reducing the pressure differential to a negative value.
The U.S. Pat. No. 4,078,856 by Thompson et al presents a general description of a light valve cell and the fluid suspension which controls the transmission of radiation through the cell.